1. Field of the Invention
The present invention relates to a mobile communication system, a handover control method, a base station, and a mobile station. More particularly, the present invention relates to a mobile communication system in which data transmission is performed from a base station to a mobile station using a shared channel.
2. Related Art
In a mobile communication system in which data is transmitted from a radio base station as an Enhanced NodeB (eNodeB) to a mobile terminal as User Equipment (UE) using a shared channel, for making a handover, the mobile terminal measures received signal levels for adjacent cells, and a radio base station which will be the source (or the origin of a handover) determines whether or not to carry out a handover based on the received signal levels of the adjacent cells as measured by the mobile terminal, and decides a radio base station which will be the destination (or the destination of a handover) (see 3GPP TR 25.813 V7.0.0 (2006-06) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Radio interface protocol aspects (Release7), 9.1.5 Handover).
Then, the source radio base station makes a handover request to the destination radio base station, and instructs the mobile terminal to undergo a handover to the destination radio base station. Also, the source radio base station starts transfer of user data sent from a core network, which is a Mobility Management Entity (MME)/User Plane Entity (UPE), to the destination radio base station.
In response to the instruction for a handover, the mobile terminal establishes synchronization with the destination radio base station and transmits a handover confirmation response to the destination radio base station, thereby completing the handover procedure. The destination radio base station notifies the source radio base station of completion of the handover procedure, and also provides the core network with location information for the mobile terminal so that the core network can directly transmit user data to the destination radio base station.
Instead of the source radio base station transferring user data to the destination radio base station at the time of a handover, it is also possible that the core network transmits user data to both the source and destination radio base stations by arranging the core network to transmit user data to any radio base station that manages a cell a pilot signal from which is received on the mobile terminal with a reception quality equal to or above a certain threshold value.
In a shared channel transmission scheme, a hard handover is conducted because it is difficult to establish temporal synchronization among cells due to the fact that packet scheduling is performed at radio base stations. Accordingly, there can be concerns such as occurrence of data loss and associated reduction in a data transmission rate. In addition, as stated above, since a radio base station decides whether to carry out a handover or not after receiving a measurement report from a mobile terminal as a procedural step of a handover, switching of radio channels takes time.
Services currently under consideration that can be provided using a shared channel include non-real-time services for data transfer, viewing of web information and the like. In such a case, when packet loss has occurred in the course of a handover, packet retransmission is carried out at a higher layer to ensure that the mobile terminal receives data. However, when a real-time service for which occurrence of delay can significantly affect QoS (Quality of Service) is provided, such as VoIP (Voice over IP) and streaming, or when high QoS is required even for a non-real-time service, retransmission control at a higher layer is not effective and delay in received data significantly affects quality of service. To realize the provision of such services on a shared channel, there is a need for a handover scheme that does not cause data loss and significant delay.